1. Field of the Invention
The invention relates generally to wireless communication devices capable of successively measuring, at variable time intervals, signal quality of a received wireless signal by the wireless communication devices, and more particularly to improvements in the techniques of controlling the lengths of the time intervals.
2. Description of the Related Art
In recent years, a wide variety of wireless communication systems (e.g., schemes, standards) have become popular, which include, for example, mobile communication systems and wireless LAN (Local Area Network) communication systems. Techniques are also known which are directed to a wireless communication device (i.e., a multi-band receiver) which uses different types of wireless communication systems simultaneously or selectively.
Such a wireless communication device, because of the use of different types of wireless communication systems, allows a current one of the communication systems to switch, if it becomes unavailable (e.g., when exiting a cell or area boundary), into an alternative one of the communication systems, resulting in undisruptive or seamless communication.
In this regard, for such a wireless communication device to achieve seamless switching between different communication systems, there is a need to periodically or successively monitor or measure, during communication under a current one of the communication systems, signal quality of a received wireless signal under an alternative one of the communication systems.
Where the signal quality is successively measured at variable time intervals Δt, if the time intervals Δt are relatively shortened, then the wireless communication device can follow rapid change in the signal quality. In this case, however, a count of times that signal quality is measured is increased, which invites an increased amount of electric power consumption.
If, however, the time-intervals Δt are relatively extended, then the wireless communication device cannot follow rapid change in the signal quality.
FIGS. 1A and 1B illustrate in functional block diagram an exemplary prior-art wireless communication device 1 capable of measuring signal quality (e.g., CINR (Carrier to Interference and Noise Ratio)) of the received wireless signal at time intervals Δt.
As illustrated in FIG. 1A, the aforementioned exemplary prior-art wireless communication device 1 is configured to include: a data transmission/reception device 100; at least one communication interface 111 (for an alternative example, communication interfaces 111 and 112); a signal-quality measurement device 121 for measuring wireless signal quality at time intervals Δt; an FIR (Finite Impulse Response) filter 122; and a time-interval controller 123 for controlling the lengths of the time intervals Δt for measurement of the signal quality.
Japanese Patent Application Publication No. 2006-332988 discloses a linear prediction technique of an SIR (Signal to Interference Ratio), using an FIR filter.
The communication interface 111, in operation, communicates with base stations or access points via a wireless network.
The signal-quality measurement device 121 measures a signal quality value at time t, which is denoted by C[t], in connection with the communication interface 111. More specifically, the signal-quality measurement device 121 measures the signal quality values C[t] at the time intervals Δt indicated by the time-interval controller 123. The measurements of the signal quality values C[t] are delivered to the FIR filter 122.
As illustrated in FIG. 1B, the FIR filter 122 performs linear prediction by convolution calculation, using a plurality of signal quality values C[t] previously obtained, to thereby estimate a future or subsequent signal-quality-value C[t].
FIG. 1B illustrates an example in which the FIR filter 122 has a tap count L=5. The FIR filter 122 sums up previous quality values obtained at previous times t−4 to t, respectively, with weights (e.g., tap weights) given to these quality values, and estimates a signal quality value at subsequent time t+1 from the sum of the weighted signal-quality-values.
The FIR filter 122 estimates a signal quality value at time t+2 by recursive entry of the estimated signal-quality-value at time t+1. Thus, the FIR filter 122 repeats calculation using previously-estimated signal quality value.
As illustrated in FIG. 1A, the time-interval controller 123 controls the lengths of the time intervals Δt which the signal-quality measurement device 121 measures the signal quality, as a function of the future signal quality value estimated by the FIR filter 122.
More specifically, in an exemplary implementation, the time-interval controller 123 determines that, if the estimated future signal-quality is at the same level as the current signal-quality, temporal changes in signal quality are slight or gentle.
In this case, the time-interval controller 123, because there is no need to monitor the signal quality frequently, extends the time intervals.
If, however, a change of the estimated future signal-quality from the previous signal quality is larger than a change of the current signal quality from the previous signal quality, the time-interval controller 123 determines that temporal changes in signal quality are instable.
In this case, the time-interval controller 123, because there is a need to monitor the signal quality frequently, shortens the time intervals.
The exemplary prior-art wireless communication device above described, however, uses an FIR filter only for the purpose of determining the lengths of the time intervals at which signal quality is measured.
Such an FIR filter would require a large amount of computation for convolution calculation. Notably, because such an FIR filter performs recursive linear-prediction, prediction errors, once occurring, would adversely affect subsequent estimation of signal quality. This can lead to significant degradation in estimation precision of future signal quality.
In view of the foregoing, it would be desirable to adaptively control the lengths of time intervals at which signal quality is measured, depending on the status of temporal changes in signal quality, without requiring an increased amount of computation for measuring signal quality.